linux-kernel - Re: [PATCH v3 1/9] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg

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Date:   Fri, 18 Nov 2022 16:15:14 +0100
From:   Heiko Carstens <hca@...ux.ibm.com>
To:     Thomas Huth <thuth@...hat.com>
Cc:     Janis Schoetterl-Glausch <scgl@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Jonathan Corbet <corbet@....net>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-s390@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Sven Schnelle <svens@...ux.ibm.com>
Subject: Re: [PATCH v3 1/9] KVM: s390: Extend MEM_OP ioctl by storage key
 checked cmpxchg

On Fri, Nov 18, 2022 at 03:37:26PM +0100, Thomas Huth wrote:
> > > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > > index 0d5d4419139a..1f36be5493e6 100644
> > > --- a/include/uapi/linux/kvm.h
> > > +++ b/include/uapi/linux/kvm.h
> > > @@ -588,6 +588,8 @@ struct kvm_s390_mem_op {
> > >   		struct {
> > >   			__u8 ar;	/* the access register number */
> > >   			__u8 key;	/* access key, ignored if flag unset */
> > > +			__u8 pad1[6];	/* ignored */
> > > +			__u64 old_p;	/* ignored if flag unset */
> > 
> > Just one comment: the suffix "_p" for pointer is quite unusual within
> > the kernel. This also would be the first of its kind within kvm.h.
> > Usually there is either no suffix or "_addr".
> > So for consistency reasons I would suggest to change this to one of
> > the common variants.
> > 
> > The code itself looks good from my point of view, even though for the
> > sake of simplicity I would have put the complete sign/zero extended
> > 128 bit old value into the structure, instead of having a pointer to
> > the value.
> 
> See
> https://lore.kernel.org/kvm/37197cfe-d109-332f-089b-266d7e8e23f8@redhat.com/
> ... it would break the "IOW" definition of the ioctl. It can be done, but
> that confuses tools like valgrind, as far as I know. So I think the idea
> with the pointer is better in this case.

Ah right, I forgot about that. Then let's do it this way.